[Bug c++/26656] New: Optimization flaw on conditionnal set of a bit.

["dominic dot quiet at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

I'm on Gentoo Linux, my the use flags affecting GCC are "fortran" and "nls". I
have tried the following with -O2. If I write a piece of code that looks like
one of these :
example 1 : a = (b == 1 ? 1 : 0);
example 2 : a = (b == 1 ? 2 : 0);
example 3 : if( b == 1 )
                a = 2;
            else
                a = 0;

The compiler optimizes it by using the sete/setne instruction instead of a
potentially slow conditional jump and then doing a bit shift if necessary.
However, while code like
example 4 : a |= (b == 1 ? 1 : 0)
example 5 : if( b == 1 )
                a |= 1;

is optimized the same way, code like 
example 4 : a |= (b == 1 ? 2 : 0)
example 5 : if( b == 1 )
                a |= 2;

is not optimized with a sete/setne, a conditional jump is used. Since there is
a workaround to make GCC use a sete/setne
workaround : t = (b == 1 ? 2 : 0);  // or if( b == 1 ) t = 2; else t = 0;
             a |= 2;

I could compare the performance of sete/setne like operations vs conditional
jumps on my AMD Ahlon XP 1700+. It turns out that, without -march=athlon-xp,
the workaround, using a if() or a ?: as constant speed that is faster than code
like example 4 and 5, except when the probability is very close to 1/2. The
further from 1/2, the slower, on the extreme, it can be more than 2x slower. At
1/2, the conditional jump is faster, but not by much. For some reason, code
like example 4 is slower than code like example 5.
With -march=athlon-xp, code like example 5 and the workaround behaves the same,
but code like example 4 as a constant time that is however slower than the
workaround but faster than most other cases when compiled without
-march=athlon-xp. Disassembly shows that it is using a conditional move.


-- 
           Summary: Optimization flaw on conditionnal set of a bit.
           Product: gcc
           Version: 3.4.5
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dominic dot quiet at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26656